1. Field of the Invention
The present invention relates to earthquake base isolation of buildings and other structures. More particularly, the invention relates to multi-step base isolators.
2. Discription of the Prior Art
The invention entitled "Earthquake Shelter" U.S. Pat. No. 4,965,895 incorporates a ball bearing base isolator where the ball bearing is supported on a pedestal plate having a concave upper surface; the ball bearing permits the superstructure to sustain a very small horizontal deformation during a severe earthquake as well as to return to its original position. But this system is not intended for resistance against wind.
In the invention entitled "Seismic Isolator" U.S. Pat. No. 4,974,378 another step is done: to prevent permanent horizontal post-earthquake offsets and at the same time to keep the system's ability to withstand wind pressure as well as minor earthquakes. A pedestal plate is provided with a depression which is shaped in compliance with the configuration of the contacting surface of the ball. In spite of substantial merits of this type of base isolation, called AFBI--Antifriction Base Isolation, under special circumstances, when the isolated structure sustains a resonant excitation with considerable amplitudes of earth displacement, the base shear can exceed windload threshold.